On Fri, 17 Feb 2006 09:43:29 -0000, "Mary Fisher"
wrote:

If you use a plastic bag as the caul, and you suck the air out of the
inside, then you can have a few psi clamping force over the whole thing,
practically for free.

And you get enough vacuum from a domestic vacuum cleaner for that? How long
does it last - doesn't air leak in?

I've never tried with a vacuum cleaner. It might well work - you only
need the crudest of vacuums. You seal it afterwards by either leaving
the pump running, or by taping over / gluegunning the hose connection.
Most pumps need some cooling air though, so the leave it running
approach only works for lab vacuum pumps. Old fridge compressors and
vacuum cleaners are known to overheat when run like this.

For degassing resin mixe I use a Vac-u-vin coffee jar with one of their
valved rubber stoppers and a hand plunger pump. You can also buy a
woodworking bagging kit made with the same pump parts.

For bigger bags I use either a hand-cranked lab vacuum pump (Griffin and
George's finest) or else a real lab vaccum pump.

It's also easier to make jigs that resitst being crushed, rather than
ones that can hold a pressure bag expanding inside them.

I don't understand that either :-(

You need to squash part A against part B, whilst holding part B in a
funny shaped curve. Putting a balllon against A would work, if you
wrapped the whole lot up inside a rigid frame. It's easier though to put
the bag around the outsides of A, B and jig, then pump the whole lot
down and squash it inwards. Compression is easier to resist than
tension.

--
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